Modulating system



y 1935. J. G. NORDAHL 2,002,209

' MODULATING SYSTEM Filed April 4, 1935 //v VENT-0R J. G. NORDAHL ZZX MA TTORNEV Patented Ma 21,1935

MODULATING SYSTEM John G. Nordahl, Orange, N. J., assignor to BellTelephone Laboratories, Incorporated, New York, a corporation of NewYork Application April 4, 1933, Serial No. 664,292 Claims. (01.179-171)This invention relates to modulating systems particularly to modulatingcircuits for radio tele phone transmitters. i

An object of the inventionis to simplify the '5 control of the poweroutput of a radio telephone transmitter.

In a specific preferred embodiment of thisinvention in a radio telephonetransmitter, the

voice or signal wave. is amplified and supplied 10 to the platecircuitof a radio frequency amplifier or oscillator tube to modulate theradio frequency Wave in the plate circuit of the tube, boththe radiofrequency and audio'frequency tubesbeing supplied with space currentfrom the same direct current source. In order to control the poweroutput of the transmitter without varying the degree or percentage ofmodulation, a

variable impedance element is included'in the common direct current andaudio frequency circuit of the modulator tube. This permits'thesimultaneous control offthe direct current and audio frequency voltagessupplied to the modulatorand consequently the simultaneous control ofthe radio and audio frequency currents. With a proper design of thecircuit elements, it ispossible by adjusting the variable impedance toso varythe audio frequency voltage supplied to the radio frequency tubein proportion to the variation in the direct current plate voltage as toper ,mit the transmitter to be adjusted for different power outputswhile maintaining the same percentage modulation.

The invention can be more readily understood by reference to thefollowing detailed description in connection with the drawing whichshows diagrammatically one embodiment of the inventtion in a radiotelephone transmitter. a

l A source of radio frequency wave I, such as a crystal controlledoscillator and buffer amplifier of theusual type, supplies radiofrequency waves tolthe grid circuit of a tube 2 of. the screen grid typewhich is operated as a modulating .amplifier. The modulated radiofrequency output of the tube 2 is applied to the grid. circuit of 45fapower amplifier tube 3 through a tuned circuit I14. and the resultantamplified energy issupplied to the transmission line ficon'nected to anantenna f.(not shown) through a tuned, output circuit 6. Q.Modu'lation"'is accomplished in t-thehmodulating "amplifier tubev 2 bysuperimposing the amplified audio frequency energy onthe direct currentvolt- Ya g essupplied to the plate and screen grid electrodes thereof.The modulating audio frequency 55. voltage is obtained ,from the audiofrequency ;,jampl ifier This amplifiercomprises twotubes 8 and 9connected in push-pull relationand having a grid circuit supplied withthe audio frequency wave from a microphone ll] through the inputtransformer H. The plates of the tubes 8 and 9 are connected to theprimary wind- 5 ing of the output transformer I2.

The cathodes of the tubesZ, 3, 8 and 9 are heated by means ofalternating current supplied from the source 13 through the transformer{4.

Two condensers I5 and I6 connectedin series 10 across the cathodecircuit and having their midpoints connected to ground and to themidpoint of the secondary winding of transformer l4 provide direct andalternating current connections to the effective midpoint of thecathodes of the 15 tubes. Space current for the audio frequencyamplifier tubes 8 and 9 and the modulator tube 2 is supplied from thebattery I! through a circuit which will be described in detail later.Grid biasing potential for the modulator tube 2 is ob 20 tained from thebattery l8 through the choke coil l9. Grid biasing potential for theaudio frequency amplifier tubes 8 and 9 is obtained from the battery 20.Space current for the power amplifier tube 3 is obtained from thebattery 2| 5 through the connection to the midpoint of the inductancecoil of the tuned circuit 6. A bypass condenser 22 provides an effectivealternating current ground connection to themidpoint of the inductanceof the tuned circuit 6 and bypasses the alternating current aroundthebattery 2|. Grid biasing potential for the power amplifier tube 3 isobtained from the battery 23 through the right-hand inductance coil ofthe tuned circuit 4; the condenser 24 providing alternating currentbypass around the battery 23, and an effective alternating currentground connection to an intermediate point of the inductancesof thetuned circuit 4. Blocking condensers 25 and 26 are provided in the tunedcircuit 4 for isolating the plate of the tube 2 from the grid of thetube 3 for direct current voltages.

the tuned coupling circuit 4. A condenser 28 is provided to balance theplate to cathode capacity of the tube 2 to keep the coupling circuit 4balanced with respect to ground. In the output cir- 0 cuit of the powertube 3 a condenser 29 is provided to similarly balance the anode tocathode capacity of the tube 3 to keep the outgoing line 5 balanced withrespect to ground.

The direct. currentspace circuit of the audio frequency amplifier tubes8 and 9 is completed I from the grounded cathode terminal throughbattery I! and the two halves of the primary windings of the transformerl2 in parallel to the anode of the tubes 8 and 9, the alternatingcurrent plate path being completed from the mid terminal of the primarywinding of transformer l2 through resistance 30 and the bypass condenser3|.

The direct current is furnished to the plate and screen grid circuits ofthe modulator tube 2 fromv the grounded cathode terminal through batteryI l, resistance 30, variable resistance 3 2; left-hand induction of thetuned coupling circuit 4 to the plate of the tube and resistance 33 tothe screen of the tube 2. Radio frequency in the 'pl'ate cir cult of themodulating tube 2 is bypassed through the condenser 34 and in the screencircuit through the condenser 35. The audio frequency voltage formodulating the carrier wave in the modulating tube 2 is supplied fromthe secondary winding 'of the transformer l2, the audio frequencycircult being completed from the lower terminal of the secondary windingof the transformer l2,

variable resistance 32 and bypass condenser 3 l to the grounded cathodeterminal. Thus modulation is attained by impressing the audio frequencyvoltage on both the plate and screen circuits of the modulatingamplifier as disclosed and claimed in thecopending application of W. L.Lawrence, Serial No. 584,099 filed December 31, 1931, Patent 1,923,53 ofAugust 22, 1933.

It will be observed that the variable resistance 32 is included in boththe direct current and audio frequency circuits of the modulator tube 2so that "an adjustment of the resistance 32 will vary the direct currentand audio frequency components of the plate and screen voltages for themodulating amplifier tube simultaneously, thus permitting the poweroutput of the transmitter to be varied while maintaining constant thepercentage modulation.

Since the efficiency of the modulating amplivfier 2 varies with thedirect current plate voltage,

the radio frequency voltage available in the plate circuit of themodulating amplifier tubev will not be a linear function of the directcurrent voltage. As aresult, it is necessary, in order to maintain thepercentage modulation constant for varying power outputs, to vary thedirect current and audiofrequency components of theplate voltage.atdifferent rates. This is accomplished by the use of the resistanceelement 30 which is included in the direct current path of the platecircuit of the modulating amplifier tube 2 but not in the audiofrequency path since it is shunted by the audio frequency bypassingcondenser 3|.

L thepercentage modulation is further facilitated by the fact that theaudio frequency amplifier "I can be designed to have an output which isinversely proportional to its load impedance. When so operated, anincrease in the resistance of the variable resistance element 32 willdecrease the audio frequency voltage applied to the modulating amplifier2 not only by reason of its voltage dividing action but also because ofthe decrease in the output of the audio frequency amplifier resultingfrom the increase in the load impedance. By taking advantage of eitheror both of these factors; namely, the effect of resistance element 30and design of the audio frequency amplifier 1, great flexibility indesign may be attained.

What is claimed is: 1. A modulating system comprising an electricdischarge device, means for impressing a carrier wave upon said device,a transformer, means for impressing a signal wave upon the primarywinding of said transformer, a source of direct current, a seriescircuit including said source of direct current and the secondarywinding of said transformer connected between the anode and cathode ofsaid device, and means included in said series circuit forsimultaneously varying at different rates the direct current voltagefrom said source and the signal frequency voltage from said transformerimpressed upon said device.

2. A modulating system comprising an electric discharge device, meansfor impressing a carrier wave upon said device, a series circuitconnected between the anode and cathode of said device, said seriescircuit including a source of signal waves for modulating said carrierwave and a source of direct current for supplying space current to saiddevice, and means including a variable resistance element forsimultaneously varying at different rates the signal current voltage andthe direct current voltage impressed upon ,said device.

3. A modulating system comprising an electric discharge device, meansfor impressing a carrier wave upon said device, a series circuitconnected between the anode and cathode of said device, said seriescircuit including a source of direct current for furnishing spacecurrent to said device and a source of signal waves for modulating thesource of direct current for supplying space current and shieldingelectrode biasing potential to said device, means for supplying carrierWaves to the input electrode of said device, a source of signalfrequency voltage for modulating said carrier waves, a circuit includingsaid source of direct current and said source of signal frequencyvoltage for supplying direct current and signal frequency voltagesbetween the cathode and anode and between the cathode and shieldingelectrode of said device, and a variable impedance elementconnected insaid circuit to simultaneously vary the direct current and signalfrequency components of the voltages supplied between said cathode andanode and between said cathode and shielding electrode,-

j 5. A modulating system comprising an electric discharge device havinga cathode, an anode and an input circuit, means for impressing carrierwaves on said input circuit, a circuit including a source of'directcurrent and a source of signal frequency waves for impressing a voltagehaving a direct current component and a signal frequency componentbetween said cathode and anode to modulate the carrier waves, and meansfor simultaneously varying the direct and signal current components ofsaid voltage in such proportions as to vary the power output of saiddevice while maintaining substantially constant the percentagemodulation of the resultant wave.

6. A modulating system comprising an electric discharge device having acathode, an anode, a control electrode and a shieldingelectrode locatedbetween the control electrode and anode, means for supplying carrierwaves between said cathode and control electrode,-a source of directcurrent, a source of signal frequency waves for modulating said carrierwaves, a circuit including said source of direct current and said sourceof signal frequency waves and having terminals connected to said anode,shielding electrode and cathode, respectively, for supplying between thecathode and anode and between the cathode and shielding electrodevoltages each having a direct current component and a signal frequencycomponent, and means including a variable impedance element connected insaid circuit for simultaneously varying said direct current and signalfrequency components in such proportions as to maintain substantiallyconstant the percentage modulation while varying the power output ofsaid electric discharge device.

7. A radio telephone transmitter comprising a source of audio waves, anelectric discharge device for amplifying audio waves from said source, asecond electric discharge device having an anode, a cathode and controlelectrode, means' for impressing carrier waves between the cathode andcontrol electrode of said second device, a source of direct current,connections from said source of direct current to the first electricdischarge device for supplying space current thereto, other connectionsfrom said source of direct current to the cathode and anode of saidsecond electric discharge device for supplying space current thereto andincluding means for impressing between the anode and cathode theamplified audio frequency output of said first electric dischargedevice, and means included in said other connections for simultaneouslyvarying the audio frequency and direct current voltages impressed uponsaid second electric discharge device in such proportion as to maintainsubstantially the perfrom said source of direct current to the firstelectric discharge device for supplying space current thereto, otherconnections from said source of direct current to said second dischargedevice for supplying direct current voltages between the cathode andanode and between the cathode and shielding electrode thereof, saidother connections including means for impressing between the cathode andanode and between the cathode and shielding electrode of said secondelectric discharge device, the amplified audio frequency output of thefirst discharge device, and having a circuit portion common to both thedirect current and audio frequency components of the voltages suppliedbetween the anode and cathode and between the shielding electrode andcathode of said second electric discharge device, and a variableresistance element connected in said circuit portion for simultaneouslyvaryingthe direct current and audio frequency components of saidvoltages in such proportions as to maintain substantially constant thepercentage modulation while varying the power output of said secondelectric discharge device.

9. A,modulating system comprising an electric discharge device having acathode, an anode and a control electrode, a cathode-control electrodecircuit therefor including a source of carrier waves, and acathode-anode circuit therefor including'a source of direct current, asource of signal frequency current, a path individual to the directcurrent, a second path individual to the signal frequency current, avariable resistance element common to the direct current and signalfrequency paths, and an impedance ele-- ment in the path individual tothe direct cur-- rent and having such a value of impedance that theratio of the resistance value of said variable resistance element to thetotal signal frequency impedance of said anode-cathode circuit isgreater than the ratio of said resistance value to the total directcurrent impedance of said circuit so that the power output of saidelectric discharge device may be varied while maintaining substantiallyconstant the percentage modulation.

10. A modulating system comprising an electric discharge device, meansfor impressing a carrier wave upon saiddevice, a series circuitconnected between the anode and cathode of said device, said seriescircuit including asource of signal waves for modulating said carrierwave and a source of direct current for supplying space current to saiddevice, and means for simultaneously varying at different rates thesignal current voltage and the direct current voltage impressed uponsaid device, said signal current voltage being varied at a greater ratethan said direct cur-: rent voltage.

JOHN G. NORDAHL.

